Silver nanowires, and production method and dispersion of the same

Abstract

Silver nanowires coated with, instead of a polymer protective agent such as PVP, an organic protective agent having a smaller molecular weight are provided. The silver nanowires have an average diameter of 100 nm or less and an average length of 5 μm or more, and a thiol having a molecular weight of 75 to 300 is adhered on surfaces of the metal silver. The silver nanowires have a thiol containing one thiol group in the structure adhered thereon. A thiol having only one thiol group (—S—H) in a molecule is a suitable target. Examples thereof include 1-dodecanethiol, 1-decanethiol, 1-octanethiol, 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid.

Description

BACKGROUND OF THE INVENTION[0001]Field of the Invention[0002]The present invention relates to silver nanowires that are useful, for example, as a material forming a transparent conductor, and a method for producing the silver nanowires. The present invention also relates to a silver nanowire dispersion in which the silver nanowires are dispersed.[0003]Description of the Related Art[0004]As used herein, “nanowires” refers to an aggregate of fine metal wires having a thickness of approximately 200 nm or less. When likened to powder, an individual wire corresponds to a “particle” constituting powder, and nanowires correspond to “powder” which is an aggregate of particles.[0005]Silver nanowires are expected as a promising conductive material for imparting electric conductivity to a transparent base material. When a transparent base material such as a glass, polyethylene terephthalate (PET), and polycarbonate (PC) is coated with a liquid in which silver nanowires are dispersed, and then ...

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Benefits of technology

[0012]Common silver nanowires coated with PVP in related arts are generally provided as a silver nanowire dispersion using an aqueous liquid medium because of good dispersibility thereof in water and other polar solvents. However, depending on the application, a silver nanowire dispersion using a solvent having small polarity or a nonpolar non-aqueous solvent is in some cases desirably used. For responding such a need, a technique in which the polymer protective agent such as PVP which has been needed for silver nanowires synthesis is substituted with a substance having good dispersibility in a liquid medium is effective.

[0014]On the other hand, in formation of a conductive network by silver nanowires, the conduction has been secured by simple contacts between the wires in related arts. If wires can be bonded at the contact points by sintering, the electric conductivity of a conductive network is considered to be greatly enhanced. However, silver nanowires having the polymer protective agent such as PVP adheres thereon have a high sintering temperature, and it is difficult in many cases to sinter the silver nanowires after applying them on a transparent base material. In particular, PVP also has a problem of easy fusing under heat.

[0016]As a result of studies, the present inventors have found an effective technique for securely substituting a polymer protective agent such as PVP with a substance having a smaller molecular weight with an easy procedure using, as raw material wires, silver nanowires obtained by a method in which silver is reduced and deposited in a wire form in an alcohol solvent in the presence of the polymer protective agent. Specifically, the above object can be achieved by adopting a thiol as the protective agent substance that is the substitute.

[0028]The liquid A and the liquid B can be mixed in the presence of an amphipathic substance such as acetone and isopropanol. In particular, when a solvent for the liquid A is a polar solvent and a solvent for the liquid B is a nonpolar solvent, the mixing under the presence of an amphipathic substance is highly effective.

[0031]In the present invention, silver nanowires having a thiol adhered on surfaces of metal silver are disclosed. On the silver nanowires immediately after synthesis through reduction and deposition, an organic substance such as a polymer used in the synthesis as a shape controller is adhered. Since thiols have high adhesiveness to metal silver, when a surfactant containing a thiol is added to the silver nanowire dispersion after the synthesis, the organic substance such as a polymer adhered on the surfaces of the silver nanowires can be substituted with the surfactant containing the thiol. By substituting an organic protective agent adhered on the silver nanowire surfaces, it becomes possible to disperse silver nanowires, which can generally be dispersed only in a polar solvent, in a nonpolar solvent such as toluene and hexane.

[0032]Monomers having a molecular weight of 300 or less are easily released from surfaces of silver nanowires at a lower temperature compared with a polymer such as PVP. For this reason, when a transparent conductive film is formed using the silver nanowires according to the present invention, through a heat treatment that is performed after a coating material containing the silver nanowires is applied on a base material, the amount of organic substances remaining on the surfaces of the nanowires can be reduced as compared with related arts. A smaller amount of organic substances on surfaces of nanowires leads to increase of probability of contacts between the nanowires and decrease of electric resistance of the conductive network. Since the density of silver nanowires in a film required for imparting a prescribed electric conductivity to a conductive film can be reduced, optical transparency is accordingly enhanced, being advantageous for forming a clear conductive film with less haze.

Problems solved by technology

However, depending on the application, a silver nanowire dispersion using a solvent having small polarity or a nonpolar non-aqueous solvent is in some cases desirably used.

However, since one molecule of the polymer protective agent such as PVP adheres to metal silver of the wires at multiple points, it is generally difficult to effectively remove the polymer protective agent through a washing operation.

It is also difficult to substitute the polymer protective agent with another surface protective agent.

However, silver nanowires having the polymer protective agent such as PVP adheres thereon have a high sintering temperature, and it is difficult in many cases to sinter the silver nanowires after applying them on a transparent base material.

In particular, PVP also has a problem of easy fusing under heat.

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